As a detector for a gas chromatograph (GC), various types of detectors have been put to practical use, such as a thermal conductivity detector (TCD), electron capture detector (ECD), flame ionization detector (FID), flame photometric detector (FPD), and flame thermionic detector (FTD). Among these detectors, the FID is most widely used, particularly for the purpose of detecting organic substances. The FID is a device that ionizes sample components in a sample gas by hydrogen flame and detects the resultant ion current. It has a wide dynamic range (the sample-concentration range within which the detection sensitivity shows an adequate linearity) of six to seven digits. However, the FID has drawbacks: firstly, its ionization efficiency is low, and therefore, the lowest detection limit is not low enough; secondly, the ionization efficiencies for alcohols, aromatic substances, and chlorine-based substances are low; and thirdly, it requires hydrogen, which is a hazardous substance, and therefore, an explosion-proof apparatus or similar kind of special equipment must be provided, which makes the entire system difficult to operate.
In recent years, a dielectric barrier discharge ionization detector (“BID”) which performs ionization by dielectric barrier discharge plasma has been put to practical use as a new type of detector for a GC (for example, see Patent Literature 1, as well as Non Patent Literature 1).
In the BID described in the aforementioned literatures, a low-frequency AC (alternate current) high voltage is applied to the discharge electrodes circumferentially provided around a dielectric quartz-glass tube, an inert gas supplied into the tube is ionized, and non-equilibrium atmospheric pressure plasma is formed. Sample components contained in a sample gas injected into the tube are ionized mainly by the effects of the light emitted from this plasma. The produced ions are collected by the collector electrode, and a detection signal corresponding to the amount of ions, namely the amount of sample components is generated. Generally, it is said that the mechanism of the ionization of sample components in the discharge ionization detector is the photoionization by high-energy vacuum ultraviolet light radiated from the plasma and the Penning ionization by meta-stable helium atoms produced by the plasma. As for the BID, as described in Non Patent Literature 1, it has been experimentally confirmed that the photoionization by the vacuum ultraviolet light mainly contributes to the ionization of the sample components.
In the aforementioned BID, the plasma is generated in a stable form, and furthermore, the quartz glass tube and other elements constituting the gas passage are not heated since the temperature of the plasma is low. Therefore, it is possible to reduce various noises due to a temporal fluctuation in the plasma, and other factors. As a result, a higher level of S/N ratio can be achieved than in the FID. The BID is also characterized by being capable of detecting a wide variety of organic and inorganic compounds with high sensitivity, which enables high-sensitivity quantitative determination of aldehydes, alcohols, halogens and other compounds for which it is difficult to obtain sufficient sensitivity with the FID.
In discharge ionization detectors including BIDs, inert gases are often used as gases for plasma generation; among these, He (helium), Ar (argon), and He with a small amount of Ar are frequently used. These gases are relatively inexpensive and have the following properties.
(1) The light energy of He plasma is very high (about 17.7 eV). Therefore, it is possible to ionize and detect most of compounds other than Ne (neon) and He. BIDs with He used as a plasma generation gas are particularly useful in detecting inorganic compounds because FIDs cannot ionize inorganic compounds.
(2) The light energy of Ar plasma is not as high as that of He plasma (about 11.7 eV). Therefore, as with FIDs, it is not possible to ionize inorganic compounds. However, this means that it is insensitive to water. Therefore, in measuring organic compounds in aqueous solution, BIDs with Ar (or He with a small amount of Ar) used as a plasma generation gas has an advantage of detecting a small amount of an organic compound with high sensitivity without being interfered by water, a solvent in a large quantity.